Are Neural Net architectures accidental discoveries?
Although there is a strong element of "try and see" that has driven successful architectures, the drivers for what to try are often inspired by underlying theory or knowledge from other disciplines.

Specifically for basic CNN, which led to AlexNet and many of the best image processing, the concept of using local receptive fields in layers was inspired by study of neurons in the cat visual system.

Modern RNNs also did not appear out of nowhere, there has long been an appreciation of the difference between a feed-forward network and a recurrently-connected one, and the different applications possible. The step change to LSTM was deliberate response to analysis of problems training simplest forms of RNN. 

Like much of science, these things are also driven by success in the real world following the research. Many promising ideas have been tried and rejected. Some have been used for a while then superseded, e.g. using RBMs or stacked auto-encoders to pre-train deep networks before ReLUs and Xavier initialisation were discovered - although both RBMs and auto-encoders still have their niches.

Tweaks to architectures, such as variants of LSTM/GRU, may even be deliberately searched and assessed as part of research. That is done with the explicit knowledge that this part of finding a good design is best done as a search across possibilities.

Despite the evolution-like progress, presenting all such advances as completely random or pure GA-like search is ignoring the conscious effort and research that leads to the designs. If you search literature on any major successful design (such as the existence of RNNs or CNNs in the first place), and read the papers, you will often find that modern neural network architectures have deep roots in older research, plus have mathematical and/or scientific justifications for the choices made.